Quartz Fiber Dosimeter - How IT Works

How It Works

The quartz fiber dosimeter is a rugged form of a device called a Lauritsen electroscope. It consists of a sealed air-filled cylinder called an ionization chamber. Inside it is a metal electrode strip that is attached to a terminal on the end of the pen for recharging. The other end of the electrode has a delicate gold-plated quartz fiber attached to it, which at rest lies parallel to the electrode. The ends of the chamber are transparent and the microscope is focused on the fiber.

During recharging, the charger applies a high DC voltage, usually around 150-200 volts, to the electrode, charging it with electrostatic charge. The quartz fiber, having the same charge, is repelled by the surface of the electrode due to the coulomb force and bends away from the electrode. After charging, the charge remains on the electrode because it is insulated.

When a particle of ionizing radiation passes through the chamber, it collides with molecules of air, knocking electrons off them and creating positively and negatively charged atoms (ions) in the air. The ions of opposite charge are attracted to the electrode and neutralize some of the charge on it. The reduced charge on the electrode reduces the force on the fiber, causing it to move back toward the electrode. The position of the fiber can be read through the microscope. Behind the fiber is a scale graduated in units of radiation, with the zero point at the position of the fiber when it is fully charged.

Since each radiation particle allows a certain amount of charge to leak off the electrode, the position of the fiber at any time represents the cumulative radiation that has passed through the chamber since the last recharge. Recharging restores the charge that was lost and returns the fiber to its original deflected position.

The charger is a small box, usually powered by a battery. It contains an electronic circuit that steps the battery voltage up to the high voltage needed for charging. The box has a fixture that requires one to press the end of the dosimeter on the charging electrode. Some chargers include a light to illuminate the measurement electrode, so that measurement, logging and recharging can occur with one routine motion.

Units with larger ranges are made by adding a capacitor attached between the electrode and the case. The capacitor stores a larger amount of charge on the device for a given voltage on the electrode. Since each radiation particle allows a fixed amount of charge to escape, a larger number of radiation particles is required to move the fiber a given amount.